Fuel injection pump

ABSTRACT

A fuel injection pump for high pressure injecting gasoline, fuel whose main component is gasoline or light oil having a sulfur content of 0.05% by weight or less by a reciprocation of a piston sliding relative to a cylinder, wherein at least a sliding surface of the piston to slide on the cylinder is made of ceramic and the surface roughness thereof, expressed in ten-point mean surface roughness R z , is 0.05-0.4 μm in a direction perpendicular to the sliding direction and 0.2-0.6 μm in a direction parallel to the sliding direction while the surface roughness expressed in the ten-point mean surface roughness R z  of a cylinder sliding surface corresponding thereto is 0.2-0.8 μm in a direction parallel to the sliding direction. By using the above combination of piston and cylinder, the wear resistance of the piston and cylinder is improved, the durability thereof is improved and the production cost can be largely reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a pump for injecting gasoline, fuel whose maincomponent is gasoline or low sulfur light oil into a combustion chamberof vehicle engine (hereinafter referred to as fuel injection pump orcalled fuel direct injection pump or direct injection pump), and moreparticularly to the same pump using wear resistant ceramics for itspiston.

2. Description of the Prior Art

Recently, an improvement of fuel efficiency of an internal combustionengine of a vehicle has been an important problem in viewpoints ofenvironment of the earth. For the reason, studies on realization ofso-called in-cylinder direct fuel injection engine in which gasoline orlight oil is directly injected into a combustion chamber of the sameengine have been made energetically. The fuel injection pump for use inthis engine utilizes a reciprocation of a cam to obtain a sufficientpressure for injecting fuel. Receiving this motion, a piston (or calledplunger) is slid reciprocatedly along an internal wall of the cylinderso as to inject fuel into the combustion chamber. This sliding conditionis very severe. Therefore, by improving the wear resistance and fatiguecharacteristic so as to reduce a wear loss, the performance anddurability of the fuel injection pump can be improved.

Particularly, the gasoline direct injection engine or diesel directinjection engine of a small passenger vehicle uses gasoline having avery low viscosity and a high volatility or low sulfur light oil havinga low sulfur content. In the same engines, lubrication of the piston andcylinder is achieved by these fuels. However, because these fuels have avery low lubricity, the insufficient wear resistance and anti-seizureperformance of the piston and cylinder may cause a fatal damage of apump.

As the countermeasure, conventionally, the configuration of the slidingsurface has been devised in various ways. For example, stainless basematerials excellent in corrosion resistance are used as a material forthe piston and cylinder and its surface is finished to be very smooth,and further to improve the wear resistance at the time of sliding,parallel grooves as shown in FIG. 1a or cross hatch lines as shown inFIG. 1b are formed in the sliding surface of the piston in a depth of2-3 μm max, so as to serve for fuel oil pit thereby increasing slidinglubricity.

According to SAE technical paper 940992, although the shallow crosshatch pattern lines are formed in the cam surface made of metal, thesurface roughness of its finished surface is 0.7 μm in terms of Rmax sothat the surface is smooth and fine. In this case, there is nodirectivity in the surface roughness so that uniform finishing isattained. In this case, the surface needs to be finished smoothly anduniformly, and therefore a very troublesome processing is needed. Thesmooth finishing of the sliding surface may be attained by honing or thelike as described in, for example, “Automobile Technical Handbook”, vol.2, page 51 and vol. 4, page 198 by Automobile Technology Association. Incase where fuel having a low sliding lubricity is used, particularly,the same treatment needs to be carried out on an internal wall of thecylinder. However, because this kind of the processing, particularly,processing on the internal wall of the cylinder is very troublesome,there is a problem in productivity.

On the other hand, about an aspect of the material, for example,Japanese Patent Application Laid-Open No.61-283759 has disclosed a casein which SIALON or a composite ceramic material of alumina and zirconia(zirconium oxide) is employed for a roller which is directly in asliding contact with a cam of a direct injection pump of a dieselengine. However, only if the sliding portion is made of ceramic, in casewhere a mating member which slides correspondingly is made of metal likesteel, its surface needs to be finished to be extremely smooth to relaxaggressiveness to the mating member and a sliding resistance relative tothe mating member. For example, Japanese Patent Application Laid-OpenNo. 8-232795 has disclosed a case of a diesel engine using low sulfurlight oil as fuel in which ceramic is employed for the sliding portionof the direct injection pump. In the same publication, the slidingsurface of the same ceramic is finished to 0.5 μm or less in terms often-point mean surface roughness Rz or 0.1 μm or less. In the abovedescribed cases, because the ceramic sliding surface having a highhardness is finished to be extremely smooth uniformly, usually in asingle direction, as described above, it takes much trouble and time forthe finishing.

Further, according to Japanese Patent Application Laid-Open No.5-340213,a conventional cheap steel material is used as a sliding portion basematerial and then smooth film is formed thereon with a hard material,such as CrN, diamond like carbon (DLC) or the like, on the slidingsurface of the base material. However, even if these materials are used,the sliding surface needs to be finished to be smooth as described aboveand additionally, there is a fear that the coated film may be peeledupon sliding. Therefore, there is a possibility that a partial seizuremay be caused thereby. As a result, there is a problem that nostabilized wear resistance or anti-seizure performance can be obtained.

SUMMARY OF THE INVENTION

The present invention has been achieved in viewpoints of these problems,and therefore it is an object of the invention to provide a fuelinjection pump of a vehicle engine using gasoline having a low slidinglubricity, fuel whose main component is gasoline or low sulfur lightoil, and more particularly to the same pump excellent in wear resistanceand anti-seizure performance at the time of sliding and which can besupplied at a low price which has not been seen conventionally,specifically a combination of its piston (plunger) and cylinder.

To achieve the above object, the present invention provides a fuelinjection pump for high pressure injecting gasoline, fuel whose maincomponent is gasoline or light oil having a sulfur content of 0.05% byweight or less by a reciprocation of a piston sliding relative to acylinder, wherein at least a sliding surface of the piston to slide onthe cylinder is made of ceramic and the surface roughness thereof,expressed in ten-point mean surface roughness R_(z), is 0.05-0.4 μm in adirection perpendicular to the sliding direction and 0.2-0.6 μm in adirection parallel to the sliding direction while the surface roughnessexpressed in the ten-point mean surface roughness R_(z), of a cylindersliding surface corresponding thereto is 0.2-0.8 μm in a directionparallel to the sliding direction.

Further, the pump having the above structure in which the ceramic is amaterial made of mainly silicon nitride or SIALON or a material made ofmainly zirconium oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1 b are diagrams schematically showing a conventionalpiston sliding surface.

FIG. 2 is a diagram schematically showing a sliding test apparatus usedin the examples of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A fuel injection pump of the present invention is a fuel injection pumpfor pressure injecting by reciprocation of a cylinder and a slidingpiston. This fuel injection pump injects fuel having a low lubricitysuch as gasoline, fuel whose main component is gasoline and light oilhaving sulfur content of 0.05% by weight or less into a combustionchamber. In a piston sliding on its cylinder, at least a sliding surfacerelative to the cylinder is made of ceramic. By forming the piston ofceramic, mutual seizure never occurs under such a severe lubricatingcondition and the wear resistance of the piston is remarkably improved.

The sliding surface of the piston of the present invention to slide onthe cylinder has a surface roughness expressed in terms of ten-pointmean surface roughness R_(z) which is 0.05-0.4 μm in a directionperpendicular to the sliding direction and 0.2-0.6 μm in a directionparallel to the sliding direction. Preferably, the surface roughness is0.1-0.3 μm in the direction perpendicular to the sliding direction and0.2-0.4 μm in the direction parallel to the sliding direction. If thesurface roughness exceeds 0.4 μm in the direction perpendicular to thesliding direction or 0.6 μm in the direction parallel to the slidingdirection, it is not desirable because the sliding surface of theopposing cylinder is worn. On the other hand, if the surface roughnessis less than 0.05 μm in the direction perpendicular to the slidingdirection or less than 0.2 μm in the direction parallel to the slidingdirection of the piston, it is not desirable because maintenance oflubricating film by fuel is insufficient and it takes much labor andtime for the processing.

In the fuel injection pump of the present invention, the surfaceroughness expressed in ten-point mean surface roughness R_(z), of thesliding surface of the cylinder which slides relative to the pistonshall be 0.2-0.8 μm in the direction parallel to the sliding direction.This reason is that if 0.8 μm is exceeded, there is a possibility that aseizure with the piston may occur at an initial phase of the sliding andadditionally the surface does not have to be finished up to as smooth asless than 0.2 μm with much labor and time. If considering economicperformance, 0.4-0.8 μm is more preferable. As the material of thecylinder, use of stainless steel is favored if considering durabilityand economic performance.

The ceramics for use in the piston of the present invention includesthose having thermal resistance (characteristic that the surface is notdeteriorated) and wear resistance under temperatures up to 150° C. whichis the highest use temperature of this kind of the pump, for example,silicon nitride (Si₃N₄) or SIALON, composite substance of SIALON andSi₃N₄, substance whose main component is silicon carbide (SiC), aluminumoxide (Al₂O₃) or zirconium oxide (ZrO₂), a composite material of thesesubstances (for example, Si₃N₄ or SIALON in which SiC is dispersed, orZrO₂ in which Al₂O₃ is dispersed) or a composite material with othercomponent than these substances(for example, Si₃N₄ or SIALON in whichTiC or TiN is dispersed). Particularly, a material whose main componentis silicon nitride (Si₃N₄) or SIALON or a material whose main componentis zirconium oxide (ZrO₂).

In case where the former is used as a piston, it is more excellent inflexural strength, hardness and wear resistance as compared to otherceramic materials and further it has a small density and light weight,so that its driving power can be reduced. Therefore, it is a favorablematerial from these viewpoints. Specifically, if silicon nitride orSIALON having 80% by weight or more is contained and the three-pointflexural strength based on JIS R1601 is at least 700 MPa, it is the mostfavorable in viewpoint of the durability.

Although the latter has a lower hardness and therefore is poorer in wearresistance, a high flexural strength can be obtained. Further, amaterial having a high thermal expansion coefficient can be obtained.The thermal expansion coefficient is about 9-11×10⁻⁶/° C. although itdepends on the composition of zirconia and this is larger as compared toother ceramics, for example, alumina of 6.5, silicon carbide of 4.7 andsilicon nitride of 3.0 in the unit of ×10⁻⁶/° C. Therefore, if the samematerial is used for the piston, in case where the cylinder is composedof steel (about 11-12×10⁻⁶/° C. although the thermal expansioncoefficient depends on the material), a difference in expansion andcontraction due to a rise or drop in temperature in a practicaltemperature range (−50-150° C.) is further decreased. Therefore, a dropin injection pressure due to leak at the time of pressure injecting offuel become unlikely to occur, and therefore, when it is combined with asteel cylinder, it is the most favorable material as the material of thepiston. In case of the piston of the present invention, it can beconsidered that its surface temperature locally rises to theaforementioned upper limit temperature of 150° C. or higher temperaturesat a practical sliding stage. Therefore, where the ceramic whose maincomponent is zirconium oxide is used and the zirconium oxide is mainlycomposed of tetragonal phase which is metastable at the ambienttemperature, it can be considered that crystal transformation fromtetragonal phase to monoclinic phase is generated by heat cycle at thepractical use time so that flexural strength and wear resistancedeteriorate. Therefore, it is favorable to use zirconium oxide in whichthe proportion of tetragonal phase is not more than 80% under judgmentby X-ray diffraction or zirconium oxide having tetragonal phase socomposed that this transformation point can be shifted to hightemperature side by adding in advance a small amount of aluminum oxide.Although cubic phase zirconium oxide or zirconium oxide containing thisis poorer in flexural strength and wear resistance than theaforementioned tetragonal phase zirconium oxide, it can be usedeffectively as the material of the piston of the present inventionbecause the cubic phase zirconium oxide is stable under hightemperatures and has not the aforementioned disadvantages accompanied bythe crystal transformation at the practical use time.

The sliding surface of the piston of the present invention to be matedwith the cylinder is finished in terms of the surface roughnessexpressed by the aforementioned ten-point mean surface roughness R_(z).to 0.05-0.4 μm in the direction perpendicular to the sliding directionand 0.2-0.6 μm in the direction parallel to the sliding direction. Asthe processing method, an ordinary centerless grinding apparatus is usedand a fine-particle diamond grinding wheel of the particle size of morethan #1000 is used to perform finish processing. Therefore, multi-stephoning processing is not required after the grinding processing unlikethe conventional steel plunger. Thus, it is possible to obtain a pistonhaving a desired surface roughness at a very low cost.

Because the aforementioned centerless grinding processing is conducted,the processing pattern is unlike a conventional cross-hatch shallowgroove. In the conventional pattern, the surface roughnesses aresubstantially the same between the directions perpendicular to andparallel to the sliding direction or the difference in surface roughnessbetween these directions is small, but in the pattern by grindingprocess, the surface roughness in the direction perpendicular to thesliding direction is usually smaller than that in the direction parallelto the sliding direction.

EXAMPLE 1

FIG. 2 is a diagram schematically showing a test apparatus forevaluating an actual pump according to this Example. In this figure,reference numeral 1 denotes a piston (plunger), numeral 2 denotes astainless made cylinder, numeral 3 denotes a cast iron cam subjected tochill hardening having four crests for reciprocating the piston 1,numeral 4 denotes a motor for driving the cam 3, numeral 5 denotes alifter for receiving and transmitting the reciprocation of the cam 3 tothe piston (plunger), numeral 8 denotes a tappet shim, numeral 6 denotesa fuel supply port, and numeral 7 denotes an exhaust port. Pistons madeof the respective materials described in Table 1 and cylinders of SUS440were prepared. In this case, the finish surface roughness of the slidingsurface of the piston to slide relative to the cylinder was madedifferent in term of the ten-point mean surface roughness between thedirection perpendicular to the sliding direction and the directionparallel to the sliding direction as shown in Table 1. The finishsurface roughness of the sliding surface of the cylinder to slide on thepiston was adjusted to substantially 0.5 μm in terms of the surfaceroughness in the direction parallel to the sliding direction. As acomparative example, a conventional piston of SUS 440 was prepared asspecimen number 1. These pistons and cylinders were installed on theaforementioned test apparatus under a combination shown in Table 1. Aordinarily sold gasoline was used as fuel and fuel injection test withan actual machine was carried out in the condition that the injectionpressure was 10 MPa and the rotation speed of the cam was 2000 rpm. Thefuel temperature during the test was 50° C. and a clearance between thepiston and cylinder was set to 3-5 μm at 50° C.

As the material for the piston, a marketed silicon nitride sintered body(described as Si₃N₄ in Table 1) having the three-point flexural strengthof 850 MPa, a marketed aluminum oxide (alumina) sintered body (shown asAl₂O₃ in Table 1) having the three-point flexural strength of 350 MPaand a marketed zirconium oxide sintered body (shown as ZrO₂ in Table 1)having the three-point flexural strength of 1100 MPa and containing 80%tetragonal phase were chosen. The sliding surfaces of these materialswere finished so as to have the surface roughnesses shown in Table 1both in the direction perpendicular to the sliding direction and thedirection parallel to the sliding direction by combining diamondgrinding wheels of #400-#1500. In this case, the finish in the directionperpendicular to the sliding direction of the piston was achieved by thecenterless grinding by changing the grinding wheel grit size. The finishin the direction parallel to the sliding direction was achieved bychanging the grinding wheel grit size and moving the specimenhorizontally in the axial direction. These finishes were carried outstep by step so as to obtain each predetermined surface roughness.

Under the above combination of the piston and cylinder, the wear lossand the absence or presence of seizure of the piston and cylinder werechecked when 200 hours and 500 hours passed since the operation wasstarted. For the piston, a difference in the outside diameter wasconfirmed and for the cylinder, a difference in the inside diameter wasmeasured and these values were shown in Table 1 as wear loss thereof.Table 2a) shows processing costs of specimen Nos. 5-7, 10-12 and Table2b) shows processing costs of specimen Nos. 20-22, 25-27. For a), theyare expressed in relative values when the specimen No. 2 is assumed tobe 1, and for b), they are expressed in relative values when thespecimen No. 17 is assumed to be 1. From the results of Tables 1 and 2,it is apparent that by using a piston having the surface finish state ofthe present invention, the wear resistance of both the piston andcylinder is improved, the durability thereof as parts is improved andthe processing cost can be largely reduced.

TABLE 1 piston surface roughness Rz Piston wear Cylinder Perpendic-Perpendic- loss wear loss ular to ular to (μm) (μm) Piston slidingsliding 200 500 200 500 No. material direction direction hours hourshours hours *1 SUS440  0.08  0.08 9.5 17.0 8.5 14.5 *2 Si₃N₄  0.02 0.10.8 1.2 2.0 3.0  3 ″  0.05 0.2 0.8 1.2 2.0 3.0  4 ″ 0.1 ″ 0.8 1.2 2.03.0  5 ″ 0.1 0.3 0.8 1.2 1.8 2.8  6 ″ 0.2 ″ 0.8 1.2 1.6 2.4  7 ″ 0.3 ″0.8 1.2 1.8 2.8  8 ″ 0.4 ″ 0.8 1.2 4.0 6.0 *9 ″ 0.5 ″ 0.8 1.2 8.0 13.0*10  ″ 0.2 0.1 0.8 1.2 1.8 3.0 11 ″ ″ 0.2 0.8 1.2 1.8 2.8 12 ″ ″ 0.4 0.81.2 1.8 2.8 13 ″ ″ 0.5 0.8 1.2 1.8 3.0 14 ″ ″ 0.6 0.8 1.2 2.5 4.5 *15  ″″ 0.7 0.8 1.2 8.5 14.0 16 ″ 0.4 0.6 0.8 1.2 4.5 6.5 *17  ZrO₂  0.02 0.11.5 2.2 2.0 3.2 18 ″  0.05 0.2 1.5 2.2 2.0 3.2 19 ″ 0.1 ″ 1.5 2.2 2.03.2 20 ″ 0.1 0.3 1.5 2.2 1.6 2.8 21 ″ 0.2 ″ 1.5 2.2 1.8 2.8 22 ″ 0.3 ″1.5 2.2 1.8 2.8 23 ″ 0.4 ″ 1.5 2.2 4.0 5.5 *24  ″ 0.5 ″ 1.5 2.2 7.5 12.0*25  ″ 0.2 0.1 1.5 2.2 1.8 3.0 26 ″ ″ 0.2 1.5 2.2 1.8 2.8 27 ″ ″ 0.4 1.52.2 2.2 3.8 28 ″ ″ 0.5 1.5 2.2 2.4 4.6 29 ″ ″ 0.6 1.5 2.2 4.2 5.5 *30  ″″ 0.7 1.5 2.2 7.5 12.5 31 ″ 0.4 0.6 1.5 2.2 4.2 5.6 32 Al₂O₃ 0.2  0.051.4 2.0 2.0 3.0 33 ″ 0.6 0.4 1.4 2.0 4.5 6.0 NOTE: *indicates acomparative example.

TABLE 2 a) b) No. processing cost No. Processing cost 5 0.6 20 0.5 3 0.521 0.4 7 0.4 22 0.3 10 0.95 25 0.9 11 0.4 26 0.3 12 0.3 27 0.2

EXAMPLE 2

The piston specimens of the specimen Nos. 5 and 20 of Example 1 andcylinders of the same material and shape as Example 1, whose slidinginternal surface to slide on the piston was finished to the ten-pointmean surface roughness shown in Table. 3 in the direction parallel tothe sliding direction were prepared. The initial number of each specimennumber of Table 3 indicates the specimen number of Example 1. With acombination shown in Table 3, the same evaluation as Example 1 wascarried out using the same test apparatus. The results are shown in thesame Table. Table 4 shows processing costs of the specimen Nos. 5-4 -5-6in relative values when the specimen No. 5-1 is assumed to be 1. Fromthe results of Tables 3 and 4, it is apparent that by using the pistonand cylinder having the surface roughness of the present invention, thewear resistance of the piston and cylinder is improved, the durabilitythereof as parts is improved and cylinder processing cost can be largelyreduced.

TABLE 3 surface roughness R_(z) in a direction Piston wear cylinder wearparallel to loss loss the cylinder (μm) (μm) sliding 200 500 200 500 No.material direction hours hours hours hours *5-1 Si₃N₄ 0.1 0.8 1.2 2.03.0  5-2 ″ 0.2 0.8 1.2 2.0 3.0  5-3 ″ 0.3 0.8 1.2 2.0 3.0  5-4 ″ 0.4 0.81.2 2.2 3.4  5-5 ″ 0.6 0.6 1.2 2.3 3.5  5-6 ″ 0.8 0.8 1.2 2.5 3.5 *5-7 ″0.9 0.8 1.2 4.0 6.5 *20-1  ZrO₂ 0.1 1.5 3.0 2.5 3.0 20-2 ″ 0.2 1.5 3.02.5 3.0 20-3 ″ 0.3 1.5 3.0 2.5 3.0 20-4 ″ 0.4 1.5 3.0 2.5 3.0 20-5 ″ 0.61.5 3.0 2.6 3.2 20-6 ″ 0.8 1.5 3.0 2.8 3.5 *20-7  ″ 0.9 1.5 3.0 4.0 6.2

TABLE 3 surface roughness R_(z) in a direction Piston wear cylinder wearparallel to loss loss the cylinder (μm) (μm) sliding 200 500 200 500 No.material direction hours hours hours hours *5-1 Si₃N₄ 0.1 0.8 1.2 2.03.0  5-2 ″ 0.2 0.8 1.2 2.0 3.0  5-3 ″ 0.3 0.8 1.2 2.0 3.0  5-4 ″ 0.4 0.81.2 2.2 3.4  5-5 ″ 0.6 0.6 1.2 2.3 3.5  5-6 ″ 0.8 0.8 1.2 2.5 3.5 *5-7 ″0.9 0.8 1.2 4.0 6.5 *20-1  ZrO₂ 0.1 1.5 3.0 2.5 3.0 20-2 ″ 0.2 1.5 3.02.5 3.0 20-3 ″ 0.3 1.5 3.0 2.5 3.0 20-4 ″ 0.4 1.5 3.0 2.5 3.0 20-5 ″ 0.61.5 3.0 2.6 3.2 20-6 ″ 0.8 1.5 3.0 2.8 3.5 *20-7  ″ 0.9 1.5 3.0 4.0 6.2

As described above, by installing the combination of the piston andcylinder of the present invention in a fuel injection pump for highpressure injecting gasoline, fuel whose main component is gasoline orlow sulfur light oil into a combustion chamber of a vehicle engine, thewear resistance at the time of sliding between the piston and cylinderis remarkably improved and provision of a cheap fuel injection pump isenabled.

What is claimed is:
 1. A fuel injection pump for high pressure injectinggasoline, fuel whose main component is gasoline or light oil having asulfur content of 0.05% by weight or less by a reciprocation of a pistonsliding relative to a cylinder, wherein at least a sliding surface ofsaid piston to slide on the cylinder is made of ceramic and the surfaceroughness thereof, expressed in ten-point mean surface roughness R_(z),is 0.05-0.4 μm in a direction perpendicular to the sliding direction and0.2-0.6 μm in a direction parallel to the sliding direction while thesurface roughness expressed in the ten-point mean surface roughnessR_(z) of a cylinder sliding surface corresponding thereto is 0.2-0.8 μmin a direction parallel to the sliding direction.
 2. A fuel injectionpump as claimed in claim 1 wherein said ceramic is a material made ofmainly silicon nitride or SIALON.
 3. A fuel injection pump as claimed inclaim 1 wherein said ceramic is a material made of mainly zirconiumoxide.